International Patent Publication No. WO 97/21570 describes a conventional braking system. This conventional braking system has a master brake cylinder, to which wheel brake cylinders are connected via wheel-brake pressure-modulation valve arrangements having a brake-pressure build-up valve and a brake-pressure reducing valve. Brake fluid released from the wheel brake cylinders through the brake-pressure reducing valves can be delivered by a return pump back in the direction of the master brake cylinder, or in the direction of the wheel brake cylinders again for renewed pressure build-up. A precharging pump is used to feed brake fluid from a reservoir to the return pump when the master brake cylinder is not actuated. Given appropriate switching of valves in the known vehicle braking system, brake pressure can be built up in the vehicle braking system both by the return pump and by the precharging pump. Furthermore, for slip control, this vehicle braking system has wheel-rotation sensors, as well as an electronic controller which evaluates signals from the wheel rotation sensors and controls the pumps and valves of the vehicle braking system. This conventional vehicle braking system has a brake-slip control, traction-control, and an operating-dynamics control device.
Methods and devices for monitoring a braking system contained in a vehicle are known. For example, German Patent Application No. 38 28 931 describes a method for monitoring the functioning of a braking system. To do so, the travel, the speed and the acceleration of the master-cylinder piston are detected by a sensor. The brake-actuation data thus detected are routed to the electronic controller. The wheel data, i.e. the wheel circumferential velocity, is detected by a wheel sensor and routed to the controller. The electronic controller checks whether the wheel data correlate to the brake-actuation data which correspond to a proper braking. If this is not the case, a pump is operated by closing a switch, the pump delivering pressurized media into the wheel brake cylinder which could not be supplied with pressurized media before because of the malfunction. However, because of its design, in this method it is only possible to monitor the braking system when it is actuated by the driver. It is not possible to monitor the braking system independently of the driver, i.e. at arbitrary points of time.
German Patent Application No. 41 18 597 describes a device for detecting and eliminating a storage-fluid leakage. The hydraulic system in which this device is incorporated includes a storage reservoir, an actuator that is actuated by fluid fed from the storage, and a control valve having an open position and closed position, respectively, for a fluid connection or a fluid shut-off between the storage and the actuator. In addition, the device includes a leakage sensor which generates an output signal indicative of a leakage flow of the fluid from the storage through the control valve in the closed position. This output signal is fed to a device, which is capable of moving the valve element of the control valve back and forth in response to the presence of the output signal. Due to the back and forth movement of the valve element, it is brought into the correct seating on the valve seat, thus ensuring a complete closure of the control valve. The fluid leakages can come about because of foreign bodies present between the valve seat and the valve element. The disadvantage of the device described above is that, besides evaluating the pump performance, in addition a pressure-operated switch is necessary for detecting the leakage.
German Patent Application No. 39 22 947 describes a hydraulic braking system which contains brake-pressure modulators that make it possible to prevent the vehicle wheels from locking as a result of brake pressures which are too high. The brake-pressure modulators include brake-line lock valves and cylinders having pistons, in response to whose displacement, pressurized media escapes from the wheel brakes to reduce the brake pressure. The piston displacements are controlled via valve configurations which are connected to a servo pressure source. Failure of the servo pressure source, if there is even a small leak of a valve arrangement, can lead to at least partial loss of braking force during a braking operation. To detect and indicate possibly existing leakiness, a control unit, acting on the valve arrangements, is adjusted, such that it controls the valve arrangements into different positions, and at the same time, observes via a pressure sensor whether logically assigned pressures are changing unacceptably. This conventional braking system also has the disadvantage that pressure sensors are necessary to implement the monitoring. It may be that it is possible to monitor the braking system independently of the driver, however the braking medium necessary for this is made available starting from a storage reservoir. Because of this, a constant pressure of the braking medium is not absolutely ensured, which can possibly lead to an impairment of the implemented monitoring.
Thus, one of the objects of the present invention is to improve conventional methods and devices, respectively, for checking or monitoring a braking system, so that, first, by using sensors (e.g., wheel r.p.m. sensors), which are present in a braking system equipped with a slip-control device, no additional sensors (e.g., pressure sensors) are necessary. Second, it is possible to monitor the braking system independently of an actuation of the braking system by the driver. At the same time, in performing the above, uniform conditions should exist (e.g., uniform pressure ratios).
A braking system according to the present invention contains at least one brake circuit. This brake circuit includes first valve configurations on the output side, each composed of a first valve, in particular an intake valve, and a second valve, in particular a discharge valve. Wheel brake cylinders allocated to the brake circuit are connected to the first valve configurations. The braking system furthermore contains a reservoir for accommodating braking medium and, on the incoming side, includes a first valve, in particular a switch-over valve, and a second valve, in particular a suction valve, the reservoir being connected to these two valves. In addition, the braking system has at least one pump which is capable of delivering braking medium to at least two wheel brake cylinders. The braking system according to the present invention has a first pump, in particular a precharging pump, which on the intake side, is directly connected to the reservoir, and which is capable of delivering braking medium in the direction of the wheel brake cylinders, and a second pump per brake circuit, in particular a return pump, which is connected to the first valve configurations in the brake circuit, and which is capable of delivering braking medium into or out of the wheel brake cylinders.
According to a method of the present invention, the at least one pump is actuated, and the first valve configurations are driven by pre-determined driving signals during predetermined vehicle states. During the actuation of the pump and the first valve configurations, a wheel-performance variable is ascertained for at least one wheel of the vehicle, the wheel-performance variable describing the wheel performance, in particular the wheel-rotation performance, of the corresponding wheel. The braking system is checked for a first and/or second fault as a function of this wheel-performance variable.
The wheel slippage and/or wheel r.p.m. and/or wheel velocity is utilized as the wheel-performance variable. One of the advantages of using the above-specified variables is that they can be ascertained using the wheel r.p.m. sensors which are at hand anyway in a braking system equipped with a slip-control (e.g., traction-control) system. Therefore, no additional sensors such as pressure sensors are necessary. Moreover, modification of the braking system, as far as the hydraulic components are concerned, is not necessary, i.e., no additional pumps or valves or other hydraulic aids are necessary to implement the method according to the present invention. In other words, it is possible to fall back on the actuators and sensors, respectively, contained in the vehicle.
Another advantage of the method according to the present invention is that a first fault and/or a second fault of the braking system is able to be ascertained using this method, i.e., selectively different individual faults or a combination of different individual faults are detectable.
Since the braking system is monitored as a function of a wheel-performance variable, the method according to the present invention therefore advantageously proceeds during predetermined vehicle states. This ensures that the wheel-performance variable, and with this variable the monitoring or checking of the braking system as well, is not invalidated by the vehicle performance. For example, the wheel-performance variable could be invalidated in response to cornering, or during an acceleration or braking process. To avoid this, the predetermined vehicle states are can be defined as follows: A predetermined vehicle state exists when the vehicle is traveling approximately straight ahead (e.g., in a straight line), and a variable describing the vehicle velocity is nearly constant, i.e., the vehicle is being neither braked nor accelerated, and the master brake cylinder is not actuated. Actuation of the master brake cylinder can be ascertained, e.g. with the aid of a brake lights switch. Such vehicle states can be brought about either intentionally, or can arise during the operation of the vehicle, i.e., during normal vehicle operation. As an example, the control at the rear end of the assembly line at the vehicle manufacturer, during which the vehicle is either moved by a driver according to defined stipulations or the vehicle is on a roller dynamometer, or a test drive after being in a service garage, could be named as intentionally caused vehicle states.
When monitoring the braking system for the first fault, a check is made as to whether or not the intake valves contained in the braking system are defective. Using this check test, the intention is to ascertain whether one of the intake valves is defectively open, i.e. whether braking medium is flowing through the intake valve, although, on the basis of its driving, it is in the blocking position. A wheel-slippage variable, which describes the wheel slippage existing at the corresponding wheel, is used as the wheel-performance variable during this check test.
All the intake valves can be brought by the predefined driving signals into a blocking position during this check test. During this driving of the intake valves, braking medium is conveyed toward the intake valves by actuating the pump. If the intake valves are in perfect condition, i.e. if in the blocking position, they in each case completely close off the associated wheel brake cylinder, then the brake pressure in the respective associated wheel brake cylinder cannot increase. On the other hand, if the intake valves do not completely close, then the brake pressure in the respective associated wheel brake cylinder will increase, which leads to a change in the associated wheel-performance variable. To better be able to ascertain the increase in brake pressure because of a defective intake valve, all the second valves of the first valve configurations, e.g., all discharge valves, can be also brought by the predefined driving signals into a blocking position. This ensures that the braking medium, which has flowed into the wheel brake cylinder, does not immediately flow out of the wheel brake cylinder again.
The check test for the first fault can be performed as follows: During the appropriate driving of the first and second valves of the first valve configurations, a wheel-performance variable is ascertained for each wheel. These wheel-performance variables are compared to a threshold value. If all the wheel-performance variables are less than the threshold value, which is synonymous with the brake pressure not having risen in the wheel brake cylinders, and the intake valves therefore being impervious, then the first fault is not present. In the event that at least one of the wheel-performance variables is greater than the threshold value, which is an indication that the first fault seems to be present, the driving of the pump and of the first and second valves is stopped.
After a predefined period of time, the pump, and the first and second valves are again driven by the predefined driving signals. The first fault exists when, in response to this repeated driving, at least one of the wheel-performance variables is once more greater than the threshold value. Therefore, to check for the first fault, the pump, and the first and second valves are driven twice, staggered over time, in order to make this check test more reliable, e.g., more rigid. The intention is for the fault to exist only when the threshold value is exceeded two times, staggered over time. It is possible to completely dispense with the second driving of the above-specified components, and that the check test for the first fault can be carried out merely with the aid of one driving. However, given this procedure, the fault check is no longer quite so rigid.
The check test for the first fault can be performed automatically from time to time during the vehicle operation, e.g., once per ignition cycle or once per operating hour, since such a fault in the intake valves can definitely occur during the operation of a vehicle due to the deposit of a dirt particle.
When monitoring the braking system for the second fault, a check is made as to whether a fault is present in the hydraulic connection of the wheel brake cylinders to the braking system, more exactly stated, to the hydraulic modulator 12, thus, in particular, wheel brake cylinders incorrectly connected (i.e., connected to an incorrect location), or whether a fault exists in the electrical connection of the wheel r.p.m. sensors to the controller, thus incorrectly connected or non-connected wheel r.p.m. sensors. The check test is preferably conducted as a test cycle. For example, the test is performed after completion of the vehicle at the manufacturer at the end of the assembly line, or after being in a service garage, since an above-described fault can occur in both cases. A wheel-velocity variable which describes the wheel r.p.m. or the velocity of the corresponding wheel is used as the wheel-performance variable during this check test.
Since the intention during this check test is to ascertain whether the wheel brake cylinders are correctly connected to the hydraulic modulator, i.e., the braking system, or whether the wheel r.p.m. sensors are correctly connected to the controller, to this end, for a first valve configuration, the predefined driving signals advantageously bring the first valve into a flow-through position and the associated second valve into a blocking position, and for the remaining first valve configurations, the first valves are brought into a blocking position, and the associated second valves are brought into a blocking position or a flow-through position. During the above-described driving of the first and second valves, braking medium is conveyed in the direction of the wheel brake cylinders by actuating the pump. Due to the above-described driving, one wheel brake cylinder is acted upon by the brake pressure built up by the pump, however the brake pressure does not act on the remaining wheel brake cylinders. Preferably, the discharge valves of these remaining wheel brake cylinders are open, to ensure that the brake pressure does not act on these wheel brake cylinders. The one wheel brake cylinder is acted upon by a brake pressure which is sufficient to bring about a noticeable change in the wheel-performance variable of the corresponding vehicle wheel, i.e., the r.p.m. or velocity of this vehicle wheel decreases significantly. No build-up in brake pressure can take place in the case of the remaining wheel brake cylinders, and thus no change can occur in the wheel-performance variable either.
The check test for the second fault can be performed as follows: During the appropriate driving of the first and second valves, a wheel-performance variable is ascertained for each wheel. The wheel-performance variable of the wheel whose first valve is brought into the flow-through position is compared to the wheel-performance variable of at least one wheel whose first valve is brought into the blocking position. In other words, the wheel-performance variable of the braked wheel is compared to an unbraked wheel. The second fault is present when the wheel-performance variable of the braked wheel does not differ to a specifiable degree from the wheel-performance variable of the at least one wheel. Thus, the wheel whose intake valve should actually be in the flowthrough position is not being acted upon by the braking medium, and thus is evidently not properly connected to the hydraulic modulator. Alternatively or additionally, the check test for the second fault can be performed as follows: For the wheel whose first valve is brought into the flow-through position, the change in the wheel-performance variable is ascertained per unit of time and compared to a predefined threshold value. The second fault exists when this change is less than the predefined threshold value.
The evaluation of the change in the wheel-performance variable has an advantage compared to the comparison of the wheel-performance variable of the braked wheel to that of the one unbraked wheel, since this evaluation can be made independently of the other wheels, thus ruling out possible error sources.
The check test for the second fault can also be performed in such a way that, in each case, the associated first valve is brought appropriately into the flow-through position for a different wheel, one after the other. In other words, the check test for the second fault is performed so that, due to the cyclical driving of the first valve configurations, the brake pressure can act upon each vehicle wheel one time during a test cycle. In so doing, sufficient time is provided in each case after the cyclical driving, so that the brake pressure of the respective wheel acted upon with pressure can again normalize.
In the event of an incorrect hydraulic connection of two or more wheel brake cylinders, or an incorrect electrical connection of two or more wheel r.p.m. sensors, the method according to the present invention permits the ascertainment of the braking of a vehicle wheel which should actually not be braked, thus making it possible to determine a connection fault.
During the actuation of the pump and of the first valve configurations, the first valve on the intake side is advantageously brought into a blocking position, and the second valve on the intake side is brought into a flow-through position. This ensures that braking medium is delivered to the wheel brake cylinders.
The method according to the present. invention can be implemented on single-circuit braking systems, just as on multiple-circuit braking systems. This method can also be implemented for hydraulic and for pneumatic braking systems. Furthermore, the method according to the present invention can be implemented for electrohydraulic or electropneumatic braking systems, in which the brake pressure is not built up by a master brake cylinder, but by a pump or by a compressor, and is controlled as a function of a brake-force signal caused by a brake pedal. The method according to the present invention can also be implemented in the case of multiple-circuit braking systems having a precharging pump for each brake circuit.
It is also possible to combine the check test for the first fault with that for the second fault. Thus, for example, the check test for the first fault can be employed within the framework of the check test for the second fault. In this manner, it is possible to rule out the recognition of a false connection of a wheel brake cylinder to the hydraulic modulator, or a false connection of a wheel-speed sensor to the controller, although an intake valve is actually defective. The above-described check tests of the braking system can also be advantageously integrated into other check test sequences.
The wheel-performance variable employed during the check test for the second fault can be used as the wheel-performance variable during the check test for the first fault. The equivalent holds true in reverse manner as well.